The present disclosure relates to developing devices used in image forming apparatuses adopting an electrophotographic system, such as copiers, printers, facsimile machines, and multifunction peripherals having combined functions of those, and to image forming apparatuses provided with such developing devices. More particularly, the present disclosure relates to developing devices that, while being supplied with fresh two-component developer containing toner and carrier, discharge surplus developer, and to image forming apparatuses provided with such developing devices.
In an image forming apparatus, an electrostatic latent image formed on an image carrier comprising a photosensitive body or the like is developed by a developing device into a toner image so as to be made visible. There is a type of developing device that adopts a two-component development system using two-component developer. In this type of developing device, two-component developer containing carrier and toner is stored in a developing container, a developing roller for feeding the developer to the image carrier is arranged, and a stirring member for feeding, while stirring and transporting, the developer inside the developing container to the developing roller is arranged.
In this developing device, as toner is consumed in developing operation, carrier remains unconsumed inside the developing container. Thus, the carrier, which is stirred with the toner inside the developing container, deteriorates due to mechanical stress from being stirred repeatedly. As a result, the carrier's ability to electrostatically charge the toner gradually diminishes.
As a solution, there have been proposed developing devices in which, while fresh developer containing carrier and toner is supplied into a developing container, surplus developer is discharged with a view to suppressing a drop in electrostatic charging performance.
For example, in a known developing device, two stirring members each having a rotary shaft and a helical blade formed in a helical shape on its circumference are arranged parallel to each other in transport chambers respectively. The transport chambers are divided by a partition portion, and in opposite end parts of the partition portion, communicating portions for delivering developer are provided. A developer discharge port is formed downstream of one of the transport chambers with respect to the developer transport direction, and between the stirring member there and the developer discharge port, an opposite helical blade formed in a helical shape wound in the direction opposite to the helical blade on the stirring member is provided, as a restricting portion, integrally with the rotary shaft.
With this developing device, when developer is supplied into the developing container, as the stirring member rotates, the developer is, while being stirred, transported to downstream of the transport chamber. As the opposite helical blade rotates in the same direction as the stirring member, the opposite helical blade applies to the developer a transporting force that acts in the direction opposite to the direction of developer transport by the stirring member. With this opposite transporting force, on the downstream side of the transport chamber, developer is blocked to have an increased height. Thus, surplus developer passes over the opposite helical blade (restricting portion) to move to the developer discharge port, and is discharged. In this way, developer can be replaced while the height of the developer inside the developing container is kept substantially constant.